Wavelength division multiplexing (WDM) systems typically comprise multiple separately modulated laser diodes at the transmitter. These laser diodes are tuned to operate at different wavelengths. When combined in an optical fiber, the WDM optical signal comprises a corresponding number of spectrally separated channels. Along the transmission link, the channels are typically collectively amplified in gain fiber, such as erbium-doped fiber and/or regular fiber, in a Raman pumping scheme. At the receiving end, the channels are usually separated from each other using thin film filter systems, to thereby enable detection by separate photodiodes.
The advantage of WDM systems is that the transmission capacity of a single fiber can be increased. Historically, only a single channel was transmitted in each optical fiber. In contrast, modern WDM systems contemplate hundreds or thousands of spectrally separated channels per fiber. This yields concomitant increases in the data rate capabilities of each fiber. Moreover, the cost per bit of data for WDM systems is typically less than comparable non-multiplexed systems. This is because any amplification system required along the link can essentially be shared by all of the separate channels transmitted in a single fiber link. With non-multiplexed systems, each channel/fiber would require its own amplification system.
Nonetheless, there are challenges associated with implementing WDM systems. First, the transmitters and receivers are substantially more complex since, in addition to the laser diodes and receivers, additional optical components are required to combine the channels into, and separate out the channels from, the WDM optical signal. Moreover, there is the danger of channel drift where the channels loose their spectral separation and overlap each other. This interferes with channel separation and demodulation at the receiving end.
In order to ensure that proper guard bands are maintained between adjacent channels and to also ensure that the carrier frequencies or wavelengths of the channels are correct both relative to other channels and relative to their wavelength assignments, optical monitoring systems are required in most WDM transmission systems. They are also useful in WDM channel routing systems, such as add/drop multiplexers and switches to ensure that the specific optical channels are being property controlled. Further, information concerning the relative and absolute powers in the optical channels is important as feedback to variable attenuators, for example.
Existing systems, however, suffer from a number of performance/complexity problems. In order to enable accurate, absolute wavelength discrimination, some reference signal source must typically be added to the signal path, filtered by the tunable filter element, and detected. This typically requires time-multiplexing and/or switching components. Moreover, there can be accuracy concerns surrounding the fact that the monitoring system is either in a calibration mode or a monitoring mode and the switching between the two can affect the operation of the tunable element in way that cannot be characterized.
The present invention is directed to an optical monitoring system. It provides for out-of-band calibration. As a result, calibration can be performed simultaneously with monitoring. This can be used to accomplish faster and/or more accurate scanning. In addition, the need for complex switching and/or multiplexing capabilities can be avoided.
In general, according to one aspect, the invention features an optical monitoring system. It comprises a signal source of an optical signal having spectrally separated channels, which are distributed within a spectral band, such as a WDM signal. A reference source generates a reference signal outside of the spectral band. A tunable filter filters the optical signal and the reference signal. A reference signal detector then detects the filtered reference signal, while an optical signal detector detects the filtered optical signal. As such, the system provides for out-of-band calibration and the concomitant advantages.
In specific embodiments, an isolator is provided to suppress back reflections into the signal source, such as a fiber pigtail endface. The reference source preferably comprises a broadband source and an etalon, which generates a reference signal with stable spectral characteristics from the broadband source""s signal. Presently, the broadband source is a super luminescent LED.
In the preferred embodiment, the filter simultaneously filters the optical signal and the reference signal by selection of the filter""s free spectral range and where the filter modes are spectrally located. This, in combination with the simultaneous detection, allows a processor to calibrate the monitoring system while monitoring the optical signal, thereby enabling more accurate and absolute spectral analysis of the optical signal.
In general, according to another aspect, the invention can also be characterized in the context of a method for optical signal monitoring. This method comprises receiving an optical signal having spectrally separated channels, distributed within a spectral band. A reference signal is also generated. The optical signal and the reference signal are then actively filtered, and the filtered reference signal and the filtered optical signal are then detected simultaneously. The spectral characteristics of the optical signal are analyzed by reference to the filtered reference signal.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.